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<head>
<meta charset="utf-8">
<meta http-equiv="description" content="javascript实现元素抛物线运动 » 张鑫旭-鑫空间-鑫生活" />
<meta name="description" content="张鑫旭web前端学习实例页面之javascript实现元素抛物线运动" />
<meta name="keywords" content="javascript, 动画, 抛物线" />
<meta name="author" content="张鑫旭, zhangxinxu" />
<title>抛物线运动</title>
<style>
body { margin: 0; font-size: 14px; font-family: 'microsoft yahei'; position: absolute; top: 0; right: 0; bottom: 0; left: 0; -webkit-user-select: none; -moz-user-select: none; user-select: none; }

p { margin: 1em; }
.target, .element { position: absolute; border: 1px solid #34538b; border-radius: 20px; }
.target { width: 100px; height: 40px; background-color: #f0f3f9; left: 480px; top: 300px; cursor: move; }
.target:active { box-shadow: inset 1px 1px 2px rgba(0,0,0,.35); }
.element { width: 30px; height: 30px; background-color: #34538b; left: 960px; top: 500px; font-size: 12px; pointer-events: none; }
.element:before { content: attr(data-center); color: #666; position: absolute; left: 100%; top: -10px; }
.target:before { content: attr(data-center); width: 100%; line-height: 40px; color: #666; position: absolute; text-align: center; }
.x { position: absolute; left: 0; top: 516px; right: 0; border-top: 1px solid #000; }
.x:before, .y:before { font-size: 40px; font-style: italic; font-family: Arial, Helvetica, sans-serif; position: absolute; }
.x:before  { content: 'x'; top: 0; right: 5px; }
.y { position: absolute; left: 976px; top: 0; bottom: 0; border-left: 1px solid #000; }
.y:before  { content: 'y'; left: 5px; top: 0; }

.article { display: inline-block; margin-left: 1em; color: #34538b; }
</style>
</head>

<body>
<p>点击屏幕任意区域开始运动，拖动椭圆目标至任意位置也能运动</p>
<div id="target" class="target"></div>
<div id="element" class="element"></div>
<i class="x" title="x轴"></i>
<i class="y" title="y轴"></i>
<a href="http://www.zhangxinxu.com/wordpress/?p=3855" class="article">本demo相关文章&raquo;</a>
<script src="lib/requestAnimationFrame.js"></script>
<script>
var funParabola = function(element, target, options) {
	/*
	 * 网页模拟现实需要一个比例尺
	 * 如果按照1像素就是1米来算，显然不合适，因为页面动不动就几百像素
	 * 页面上，我们放两个物体，200~800像素之间，我们可以映射为现实世界的2米到8米，也就是100:1
	 * 不过，本方法没有对此有所体现，因此不必在意
	*/
	
	var defaults = {
		speed: 166.67, // 每帧移动的像素大小，每帧（对于大部分显示屏）大约16~17毫秒
		curvature: 0.001,  // 实际指焦点到准线的距离，你可以抽象成曲率，这里模拟扔物体的抛物线，因此是开口向下的
		progress: function() {},
		complete: function() {}
	};
	
	var params = {}; options = options || {};
	
	for (var key in defaults) {
		params[key] = options[key] || defaults[key];
	}
	
	var exports = {
		mark: function() { return this; },
		position: function() { return this; },
		move: function() { return this; },
		init: function() { return this; }
	};
	
	/* 确定移动的方式 
	 * IE6-IE8 是margin位移
	 * IE9+使用transform
	*/
	var moveStyle = "margin", testDiv = document.createElement("div");
	if ("oninput" in testDiv) {
		["", "ms", "webkit"].forEach(function(prefix) {
			var transform = prefix + (prefix? "T": "t") + "ransform";
			if (transform in testDiv.style) {
				moveStyle = transform;
			}
		});		
	}
	
	// 根据两点坐标以及曲率确定运动曲线函数（也就是确定a, b的值）
	/* 公式： y = a*x*x + b*x + c;
	*/
	var a = params.curvature, b = 0, c = 0;
	
	// 是否执行运动的标志量
	var flagMove = true;
	
	if (element && target && element.nodeType == 1 && target.nodeType == 1) {//element.nodeType == 1表示元素 2属性 3文本 8注释 9文档
		var rectElement = {}, rectTarget = {};
		
		// 移动元素的中心点位置，目标元素的中心点位置
		var centerElement = {}, centerTarget = {};
		
		// 目标元素的坐标位置
		var coordElement = {}, coordTarget = {};
		
		// 标注当前元素的坐标
		exports.mark = function() {
			if (flagMove == false) return this;
			if (typeof coordElement.x == "undefined") this.position();
			element.setAttribute("data-center", [coordElement.x, coordElement.y].join());
			target.setAttribute("data-center", [coordTarget.x, coordTarget.y].join());
			return this;
		}
		
		exports.position = function() {

			if (flagMove == false) return this;
			
			var scrollLeft = document.documentElement.scrollLeft || document.body.scrollLeft,
				scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
			
			// 初始位置
			if (moveStyle == "margin") {
				element.style.marginLeft = element.style.marginTop = "0px";
			} else {
				element.style[moveStyle] = "translate(0, 0)";
			}
			
			// 四边缘的坐标
			rectElement = element.getBoundingClientRect();
			rectTarget = target.getBoundingClientRect();
			
			// 移动元素的中心点坐标
			centerElement = {
				x: rectElement.left + (rectElement.right - rectElement.left) / 2 + scrollLeft,
				y: rectElement.top + (rectElement.bottom - rectElement.top) / 2	+ scrollTop
			};
			
			// 目标元素的中心点位置
			centerTarget = {
				x: rectTarget.left + (rectTarget.right - rectTarget.left) / 2 + scrollLeft,
				y: rectTarget.top + (rectTarget.bottom - rectTarget.top) / 2 + scrollTop		
			};
			
			// 转换成相对坐标位置
			coordElement = {
				x: 0,
				y: 0	
			};
			coordTarget = {
				x: -1 * (centerElement.x - centerTarget.x),
				y:  -1 * (centerElement.y - centerTarget.y)	
			}; 
			
			/*
			 * 因为经过(0, 0), 因此c = 0
			 * 于是：
			 * y = a * x*x + b*x;
			 * y1 = a * x1*x1 + b*x1;
			 * y2 = a * x2*x2 + b*x2;
			 * 利用第二个坐标：
			 * b = (y2+ a*x2*x2) / x2
			*/
			// 于是
			b = (coordTarget.y - a * coordTarget.x * coordTarget.x) / coordTarget.x;	
			// console.log(this);
			return this;
		};		
		
		// 按照这个曲线运动
		exports.move = function() {
			// 如果曲线运动还没有结束，不再执行新的运动
			if (flagMove == false) return this;
			
			var startx = 0, rate = coordTarget.x > 0? 1: -1;

			var step = function() {
				// 切线 y'=2ax+b
				var tangent = 2 * a * startx + b; // = y / x
				// y*y + x*x = speed
				// (tangent * x)^2 + x*x = speed
				// x = Math.sqr(speed / (tangent * tangent + 1));
				startx = startx + rate * Math.sqrt(params.speed / (tangent * tangent + 1));
				
				// 防止过界
				if ((rate == 1 && startx > coordTarget.x) || (rate == -1 && startx < coordTarget.x)) {
					startx = coordTarget.x;
				}
				var x = startx, y = a * x * x + b * x;
				
				// 标记当前位置，这里有测试使用的嫌疑，实际使用可以将这一行注释
				element.setAttribute("data-center", [Math.round(x), Math.round(y)].join());
				
				// x, y目前是坐标，需要转换成定位的像素值
				if (moveStyle == "margin") {
					element.style.marginLeft = x + "px";
					element.style.marginTop = y + "px";
				} else {
					element.style[moveStyle] = "translate("+ [x + "px", y + "px"].join() +")";
				}
				
				if (startx !== coordTarget.x) {
					params.progress(x, y);
					window.requestAnimationFrame(step);	
				} else {
					// 运动结束，回调执行
					params.complete();
					flagMove = true;	
				}
			};
			window.requestAnimationFrame(step);
			flagMove = false;
			
			return this;
		};
		
		// 初始化方法
		exports.init = function() {
			this.position().mark().move();
		};
	}
	
	return exports;
};

// 这是很简单的拖拽方法，与本demo主旨无关，方便演示使用
var funDrag = function(element, callback) {
	callback = callback || function() {};
	var params  = {
		left: 0,
		top: 0,
		currentX: 0,
		currentY: 0,
		flag: false
	};
	//获取相关CSS属性
	var getCss = function(o,key){
		// console.log(o);
		// console.log(key);
		// console.log( o.currentStyle[key]);
		console.log(document.defaultView.getComputedStyle(o,false)[key]);
		return o.currentStyle? o.currentStyle[key] : document.defaultView.getComputedStyle(o,false)[key]; 	
	};
	
	//拖拽的实现
	if(getCss(element, "left") !== "auto"){
		params.left = getCss(element, "left");
	}
	if(getCss(element, "top") !== "auto"){
		params.top = getCss(element, "top");
	}
	//o是移动对象
	element.onmousedown = function(event){
		params.flag = true;
		event = event || window.event;
		params.currentX = event.clientX;
		params.currentY = event.clientY;
	};
	document.onmouseup = function(){
		params.flag = false;	
		if(getCss(element, "left") !== "auto"){
			params.left = getCss(element, "left");
		}
		if(getCss(element, "top") !== "auto"){
			params.top = getCss(element, "top");
		}
		callback();
	};
	document.onmousemove = function(event){
		event = event || window.event;
		if(params.flag){
			var nowX = event.clientX, nowY = event.clientY;
			var disX = nowX - params.currentX, disY = nowY - params.currentY;
			element.style.left = parseInt(params.left) + disX + "px";
			element.style.top = parseInt(params.top) + disY + "px";
		}
	}	
};


/* 元素 */
var element = document.getElementById("element"), target = document.getElementById("target");

// 抛物线元素的的位置标记
var parabola = funParabola(element, target).mark();
// 拖拽 
funDrag(target);
// 抛物线运动的触发
document.body.onclick = function() {
	element.style.marginLeft = "0px";
	element.style.marginTop = "0px";
	parabola.init();
};

</script>
</body>
</html>